METHOD FOR CHLORINE-FREE BLEACHING OF PULP WITH ACETIC ACID AS ACIDIFYING AGENT

Abstract: A method of bleaching pulp of cellulosic fibre material without the use of chlorine when manufacturing chemical pulp is described, wherein the pulp entering is fed continuously in a bleaching line and is treated with a sequestering agent in the presence of an acid in order to adjust the pH to a value below 7 and is thereafter bleached in a sequence of several stages consisting of peroxide, at least one organic per-acid and peroxide. According to the invention it is suggested that in the treatment with the sequestering agent the pulp is acidified with acetic acid.

Description: Method for chlorine-free bleaching of pulp with acetic acid as acidifying agent. The present invention relates to a method of bleaching pulp of cellulosic fibre material without the use of chlorine when manufacturing chemical pulp, wherein the pulp entering is fed continuously in a bleaching line and is treated with a sequestering agent in the presence of at least one acid in order to adjust the pH to a value below 7 and is thereafter bleached in a sequence of several stages consisting of peroxide, one or more organic per-acids and peroxide. The strict environmental requirements concerning the use of bleaching agents containing chlorine for bleaching pulp have resulted in extensive research work to find other bleaching agents that are not detrimental to the environment in the way the chlorine-containing bleaching agents are which form chlorinated organic compounds. Thus, it has been proposed to bleach pulp using ozone and peroxide in various sequences preceded by treatment with sequestering agent such as those described in SE-A- 9101300-3, for instance. However, the use of ozone involves a number of drawbacks. Ozone has a decomposing effect on the cellulose, and the equipment for producing and supplying ozone in connection to the bleaching process is vast and expensive. US-A-3,867, 246 corresponding to SE-B-405130 describes another bleaching process without the use of chlorine, including the sequence peroxide, peracetic acid and peroxide, the first peroxide stage being preceded by treatment with sequestering agent in the presence of an acid in order to adjust a low pH value. The acid used is generally sulphuric acid. However, like ozone, also peracetic acid has a strongly decomposing effect on the cellulose. The object of the present invention is to solve the drawback in bleaching pulp without the use of chlorine, using peracetic acid in combination with peroxide in the bleaching sequence PPaP preceded by treatment with a sequestering agent (Q) in the presence of an acid, so that a ready-bleached pulp is obtained that has acceptable viscosity and strength properties and a brightness of 85-90% ISO requested by the market. The method according to the invention is characterised in that during the treatment with the sequestering agent the pulp is acidified with acetic acid. The pulp entering is preferably produced by means of a digestion process giving a low Kappa number, and is thereafter subjected to delignification with oxygen gas to that the pulp obtains a Kappa number of about 12 or less and a viscosity of at least about 1000 dm^/kg. The peroxide used is preferably hydrogen peroxide. The second bleaching stage, following the first peroxide stage, is performed using performic acid, peracetic acid or perpropionic acid, preferably peracetic acid or a mixture of at least two of said per-acids. According to a suitable embodiment acetic acid is added in such an amount that the pH value is below 6, preferably below 5. The temperature during all bleaching stages should be 80°C or above, preferably 90°C. Each peroxide stage is suitably performed in alkaline solution. The peracetic acid stage is preferably performed in acid solution. Acetic acid and peracetic acid, or other suitable per- -acids are preferably manufactured on site. The consistency of the pulp during the various treatment stages is about 8-16%, preferably 9-11%. The following example illustrates the invention further and shows its unexpected results. Example A number of experiments were performed using a sulphate pulp of softwood which had been delignified with oxygen gas and washed. The delignified pulp had a Kappa number of 12.1, brightness 33.7% ISO and a viscosity of 1020 dm^/kg. It was supplied into the bleaching line in a consistency of 10% and this was maintained during the treatment with the sequestering agent EDTA and all the bleaching stages. In experiments 1 and 2 sulphuric acid was used as acidifying agent in the Q-stage, whereas acetic acid was used in experiments 3 and 4 in accordance with the present invention. Experiments 1-4 Additives Bleaching sequence Conditions OPPaP OPPaP 3. Result 3 4 O-staσe 1. EDTA, kg/BDMT 2.0 2.0 H2SO , kg/BDMT 6.2 Acetic acid, kg/BDMT 15.0 2. Initial pH value 4.7 4.8 Time, min. 60 60 Temperature, °C 70 70 P-staσe 1. H 0 , kg/BDMT 35 35 MgS0 4 , kg/BDMT 3.0 3.0 NaOH, kg/BDMT 25 25 2. Time, min 240 240 Temperature,°C 90 90 Final pH value 10.9 11.0 3. Consumed H 2 0 2 , kg/BDMT 18 16 Kappa number 5.1 4.9 Viscosity, dm 3 /kg 910 911 Brightness, % ISO 77.3 77.0 Pa-stage 1. Peracetic acid calculated as total H 2 0 2 , kg/BDMT 20 30 20 30 2. pH, buffer solution (sodium acetate) 4.3 4.3 4.3 4.3 Time, min. 240 240 240 240 Temperature, °C 90 90 90 90 Cont. 1. Additives Bleaching sequence 2. Conditions OPPaP OPPaP 3. Result 1 2 3 4 3. Consumed peracetic acid as H 2 0 , kg/BDMT 11 . 6 17. 9 9 . 9 14. 2 Kappa number 1 . 1 0. 7 1 . 2 0. 9 Viscosity, dm 3 /kg 842 814 852 841 Brightness, % ISO 84. 4 87. 4 84.4 86.8 P-staςre 1. H 2 0 2 , kg/BDMT 4.0 4.0 4.0 4.0 MgS0 , kg/BDMT 3.0 3.0 3.0 3.0 NaOH, kg/BDMT 7.0 7.0 7.0 7.0 2. Time, min. 180 180 180 180 Temperature, °C 90 90 90 90 Final pH value 10.3 10.4 10.6 10.5 3. Consumed H 2 0 2 , kg/BDMT 1.5 1.3 0.9 1.1 Kappa number 0.8 0.5 1.1 0.7 Viscosity, dm 3 /kg 808 778 834 828 Brightness, % ISO 88,1 89,3 87,8 89,2 The results show that surprisingly the pulps pre-treated and bleached in accordance with the invention have significantly improved viscosity, which in turn indicates improved strength properties as compared with the pulps pre-treated and bleached according to Examples 1 and 2. Using equal quantities of peracetic acid (Pa) in the second bleaching stage, 20 kg in Examples 1 and 3 and 30 kg in Examples 2 and 4, the use of acetic acid instead of sulphuric acid as acidifying agent in the Q-stage results in an increase in viscosity by 26 and 50 units, respectively. As mentioned above, acetic acid and peracetic acid or other suitable per-acids are preferably produced on site. Acetic acid, for instance, can be recovered from the black liquor in quantities up to 50 kg per ton of pulp when manufacturing pulp. Peracetic acid, for instance, can be produced in known manner from the acetic acid recovered. Consequently the method according to the invention can be performed in an extremely advantageous manner from the economical point of view.

Claims: C L A I M S 1. A method of bleaching pulp of cellulosic fibre material without the use of chlorine when manufacturing chemical pulp, wherein the pulp entering is fed continuously in a bleaching line and is treated with a sequestering agent in the presence of at least one acid in order to adjust the pH to a value below 7 and is thereafter bleached in a sequence of several stages consisting of peroxide, one or more organic per-acids and peroxide, characterized in that in the treatment with the sequestering agent the pulp is acidified with acetic acid. 2. A method as claimed in claim 1, characterized in that the peroxide used is hydrogen peroxide. 3. A method as claimed in claims 1 or 2, characterized in that the one or more organic per-acids used is/are performic acid, peracetic acid or perpropionic acid, preferably peracetic acid or a mixture of at least two of said per-acids. 4. A method as claimed in any of claims 1-3, characterized in that acetic acid, is added in such an amount that the pH value is below 6, preferably below 5. 5. A method as claimed in any of claims 1-4, characterized in that each peroxide stage is performed in alkaline solution. 6. A method as claimed in any of claims 1-5, characterized in that the consistency of the pulp during the treatment with sequestering agent and during said bleaching sequence is about 6-16%, preferably 9-11%. 7. A method as claimed in any of claims 1-6, characterized in that the pulp entering has been treated with oxygen gas and has a Kappa number of about 14 or less, preferably 12 or less and most preferably 10 or less. 8. The use of acetic acid, as acidifying agent when treating a chemical pulp with a sequestering agent in the bleaching of pulp without the use of chlorine, wherein the pulp entering is fed continuously in a bleaching line and is treated with a sequestering agent in the presence of acetic acid, in order to adjust the pH value to a value below 7, and is thereafter bleached in a sequence of several stages consisting of peroxide, one or more organic per-acids and peroxide.

Inventors: NILSSON ERIK, TIBBLING PETTER, NILSSON, ERIK, TIBBLING, PETTER
Applicants: KAMYR AB, NILSSON ERIK, TIBBLING PETTER, KAMYR AKTIEBOLAG, NILSSON, ERIK, TIBBLING, PETTER
Application year: 1994
Main classification: D21C9
IPC codes: D21C9